1. Field of the Invention
The present invention relates to a machine for removing contaminants from a fiber suspension, and, more particularly, to a flotation machine used for removing contaminants from a fiber suspension.
2. Description of the Related Art
A paper-making machine receives a prepared fiber suspension and produces a fiber web, such as a paper web. The fiber suspension is prepared from a source of fiber, such as wood fiber, within a stock preparation system. The stock preparation system may include a flotation machine which is used to float contaminants to the top of the suspension for removal thereof. Typically, the fiber suspension is at a relatively high temperature as a result of the various mechanical forces which are applied thereto and chemical reactions which occur therein. Contaminants within the fiber suspension in the form of particulates, such as varnish particles, dirt, etc. are relatively unaffected by the higher temperature of the fiber suspension and may be adequately floated to the top of the fiber suspension for removal. However, contaminants within the fiber suspension may also be in the form of various waxes which are contained within recycled paper. The waxes are transformed into colloidal particles within the fiber suspension at the higher temperatures at which the fiber suspension is typically transported into the flotation machine. The density of the colloidal wax particles at the temperature of the fiber suspension does not allow the wax particles to be effectively removed from the fiber suspension.
Waxes in the form of petroleum waxes have long been used in the paper converting industry to provide cellulosic paper products moisture resistance and wet strength. These paper products include corrugated boxes, paper cups, waxed paper wrapper and waxed roll-wrap laminates. Of these products, corrugated boxes account for the majority of the wax consumed. When wax is applied to corrugated boxes, the main objective is to provide board structural integrity in humid and wet environments. Examples of wax corrugated boxes are poultry boxes, vegetable boxes and fresh fruit boxes.
The majority of wax used in the corrugated board plant is paraffin wax, a by-product of the automotive lubricating oil refining process. Paraffin wax is a crystalline material composed primarily of a mixture of hydrocarbon molecules. Solid paraffin waxes are usually white, translucent tasteless and odorless. In a molten state, paraffin waxes are clear, colorless liquids of low viscosity. The melting point of paraffin waxes ranges from 43-71xc2x0 C.
Wax is applied to corrugated paperboard in three ways: a) Impregnating, by dipping or submerging paperboard into a bath of molten wax; b) Cascading, by flushing a thick xe2x80x9cwaterfallxe2x80x9d of wax into or over the finished board; and c) Curtain-coating, by passing the paperboard horizontally through a thin curtain of falling wax. The curtain-coated wax is usually a mixture of paraffin wax and other non-wax additives such as resins, rosins, polymers, and copolymers.
Waxed old corrugated container (OCC), entering through the collection process, is the largest source of wax contamination in paper recycling mills. These waxes present a major obstacle to the paper recycling industry. The problems with wax for papermaking are as follows: a) Presence of wax in a paper sheet results in the reduction of coefficient of friction, which in turn causes troubles in rewinding, converting, and box stacking; b) Waxes tend to deposit on the surface of fibers, thus lowering the interfiber bonding strength. The paper and board made from such fibers possess lower strength properties when the amount of wax in the paper and board reaches 0.5%. Also, there is some loss in stiffness; c) Paper and board containing wax generally have poor glueability; d) Non-dispersed waxes (such as curtain coating) introduce unpleasant spots in paper sheets during drying; and e) Occasionally waxes can aggravate a pitch problem. Wax tends to soften the pitch, thus making it more tacky.
Low-viscosity waxes (used for cascading or impregnating processes) are easily detached and dispersed during pulping at or above the melting point of wax. The dispersed wax forms an emulsion with water. The average size of wax particles in the emulsion is about 5 xcexcm. At this size range, conventional mechanical separation equipment, such as screen and cleaner, is not effective in removing these contaminants.
Because of the above-mentioned problems with wax, waxed OCC and other waxed paper products have been classified as non-recyclable. As such, waxed OCC is generally sorted out from the recycling plant and incinerated or landfilled. In order to utilize waxed OCC and other waxed paper products as a fiber resource for papermaking, removal of wax from the pulp suspension it necessary.
It is known to inject gas at a lower temperature into a fiber suspension within a flotation machine to clump the wax particles within the fiber suspension and thereby allow effective flotation and removal of the wax particles. However, depending upon various operating parameters such as the volumetric flow rate of the fiber suspension within the flotation machine, volumetric flow rate of the low temperature gas injected into the flotation machine, temperature of the fiber suspension and temperature of the low temperature gas, the temperature reduction of the fiber suspension within the flotation machine may not be sufficient to allow adequate clumping of the wax particles within the fiber suspension. Thus, although such a flotation machine may be adequate to remove wax for some applications, it may not provide adequate wax removal for other applications. A flotation machine which injects cold air into the fiber suspension is disclosed in U.S. patent application Ser. No. 09/595,214, entitled xe2x80x9cFLOTATION MACHINE FOR A FIBER SUSPENSION AND METHOD OF USING SAMExe2x80x9d, which is commonly assigned to the assignee of the present invention.
What is needed in the art is a flotation machine and corresponding method of operation which allow effective removal of wax contaminants within the fiber suspension.
At The present invention provides a flotation machine which injects high temperature gas into the fiber suspension to decrease the density of the colloidal wax and thereby allow the wax to be floated to the top and removed.
The invention comprises, in one form thereof, a flotation machine for removing a contaminant from a fiber suspension, including a flotation cell having an inner chamber. At least one gas injector injects a gas into the fiber suspension within the inner chamber at a temperature above 100xc2x0 C. to decrease the density of the wax within the fiber suspension and float the wax to the top of the fiber suspension. A wax removal device includes an elongate element positioned at and configured to move along a top of the inner chamber to remove contaminants from the fiber suspension.
The invention comprises, in another form thereof, a method of removing wax from a fiber suspension. A flotation machine is provided which includes at least one flotation cell having an inner chamber. The fiber suspension is transported into the inner chamber. A gas is injected into the fiber suspension within the inner chamber at a temperature above 100xc2x0 C. to decrease the density of the wax within the fiber suspension and float the wax of decreased density to the top of the fiber suspension. The wax is removed from the fiber suspension.
An advantage of the present invention is that the density of the wax within the fiber suspension is decreased, thereby allowing flotation and removal thereof.
Another advantage is that the floated wax may be removed with different types of removal devices.